Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressures.
Usually natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stock suitable for liquefying at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled (to reduce flashed vapour through one or more expansion stages) to final atmospheric pressure suitable for storage and transportation. The flashed vapour from each expansion stage can be used as a source of plant fuel gas.
The costs in creating and running a liquefying natural gas (LNG) plant or system are naturally high, and much is for the cooling configurations. Thus any reduction in the energy requirements of the plant or system has significant cost benefit. Reducing the cost of the cooling configuration is particularly advantageous.
U.S. Pat. No. 4,541,852 is directed to a base load LNG system, and shows a slip stream of feed natural gas which is reintroduced into the liquefied natural gas stream after the liquefied natural gas stream is reduced in pressure through a valve. This has the problem of not fully utilising available work from the feed natural gas.